1. Field of the Invention
The present invention relates to a chain roller structure in a chain driven vehicle such as a motorcycle.
2. Description of Background Art
A chain roller structure in a motorcycle is described in Japanese Utility Model Publication No. 60-60388 entitled "Slack Prevention Device for Drive Chain in Motorcycle", for example. According to FIG. 2 in this publication, the prior art chain roller structure includes a swing arm 16 pivotably supported at its one end to a body of the motorcycle, a roller 15 rotatably supported to the other end of the swing arm 16, a fixed arm 21 located below the swing arm 16 and welded to the body, and an adjusting bolt 23 mounted to the fixed arm 21 in perpendicular relationship to the swing arm 16 so that an upper portion of the adjusting bolt 23 abuts against the swing arm 16. The roller 15 functions to apply an upward force to a drive chain 1, thereby preventing slack of the drive chain 1.
The roller 15 is generally formed of a hard resin in consideration of wear resistance, resulting in poor shock absorptivity.
In a kind of motorcycle called an off-road motorcycle, there is a possibility of hopping (large vertical movement) upon deceleration during running on an unpaved rough road such as a mountain road. At this time, tension is cyclically applied to the chain 1 in a driving direction and a decelerating direction, and this chain tension becomes higher than that during normal running. The cycle of generation of such a high chain tension is about 15 times per second.
When the chain tension acts on a lower portion of the chain (in the decelerating direction) and the lower portion comes into contact with the roller 15 located below the chain, a force of contracting a rear suspension is generated. Such an impactive chain tension intermittently generated has an adverse effect on the rear suspension to possibly deteriorate ride comfort.